A complementary metal oxide semiconductor (CMOS) image sensor is a key component of many digital video cameras and other “high tech” devices. The CMOS image sensor is typically comprised of an upper stack that includes one or more layers of color filters and a microlens array, and a lower stack that includes interlevel dielectric layers, interlevel metal layers, and passivation layers that are formed on a substrate. The function of the microlens component is to focus incident light through a light column onto a sensing area (photodiode) at the base of the lower stack. The elementary unit of the image sensor is a pixel which is an addressable area element with intensity and color attributes related in large part to the spectral signal contrast obtained from the photon collection efficiency of the microlens array, light transmission through the color filters, microlenses, and other layers in the imaging path, and the spectral response and efficiency of the photodiode. A pixel converts incident radiation into a quantity of electrical charge that is related to the intensity of illumination. Output signals from a plurality of pixels are used by the image sensor device to generate a picture.
A plurality of pixels forms an array on the substrate wherein pixels sensitive to red light, blue light, or green light are evenly distributed. Ideally, all pixels of a certain color should afford the same output in terms of electrical charge when exposed to the same intensity of incident light. However, process variations during CMOS image sensor fabrication and other factors such as particle defects on the surface of the image sensor cause the output of certain pixels to vary either above or below the desired output range. If a large enough number of the pixels in the array fail to provide an acceptable output signal, the image sensor is rejected. Therefore, pixel arrays are typically tested while still on the chip and before the image sensor is incorporated into a larger device.
Testing is a major cost component of the final image sensor device because of the large number of pixels that must be tested, and the nature of the test which includes both optical and electrical methods. One important test is the so called photon transfer curve (PTC) test where the light sensitivity of an array of pixels in response to incident light is determined. The PTC characterizes the image sensor in many different ways. In order to calculate the read noise, the dynamic range, conversion gain, offset, offset fix pattern noise and the full well, it is necessary to grab frames with different exposure times at a certain illumination. The exposure time or integration time is usually named in number “s” of rows. One row exposure is equivalent to the number of pixels per row multiplied by clock frequency in microseconds. The illumination is chosen in a way such that the sensor reaches saturation (white image, maximum output level) at the longest exposure time.
During a conventional PTC test, a measured amount of broad band light that is highly uniform over the entire exposure field is directed at the pixel array through a point light source. Each pixel has a photo gate (photodiode) at the bottom of its light column that is pre-charged to a certain voltage level. During the time that pixels are exposed, the light photons discharge the photo gate and the intensity of light incident on the photo gate is related to the amount of discharge. The remaining voltage is transferred to a storage node (capacitor) where the voltage stays until the “pixel information” (voltage level) is read out. The read out time is related to exposure time. Normally, the test involves 5 to 10 different illumination levels (different illumination times) and each illumination requires two frames wherein a frame is defined as a certain number of lines (rows) of pixels from an array that consists of “m’ rows and “n” columns of pixels. A typical exposure rate of 30 frames per second and the wait times between light intensity changes dictate that the total PTC testing time is about 6 seconds per device. Note that the sequence of illumination, voltage transfer to storage, and read out of data to a frame grabber and ultimately to an image processor can be performed simultaneously for different frames.
Since common light sources are not programmable and fast switching, it is generally not feasible with current test methods to vary the illumination (photon flux) during the frame. Changing the intensity of the light source from one illumination to the next is typically less accurate than changing the illumination level by varying the time for a fixed illumination. As a result, improvements in PTC and dark current test throughput are limited because of the multiple number of frames required in the standard test method. Therefore, a faster method of testing image sensors and one that has improved result stability is needed to reduce the cost associated with device fabrication.
In U.S. Pat. No. 6,774,893, a method for calibrating an array of light sources involves sequentially activating each light source, sampling light intensity to generate a plurality of intensity signals, and generating a normalized matrix of the plurality of signals.
U.S. Patent Application 2005/0001142 describes a regulation system for an optical sensing device that comprises timing the duration of an integration period or determining the rate of evolution of the integration signal, comparing this duration or rate of evolution with at least one reference value, and controlling the power of the light source as a function of the result of the comparison.
U.S. Pat. No. 5,594,273 discloses using a programmable light source in a stepper to program IC dies with identification codes.
U.S. Patent Application 2006/0038916 describes an “intelligent” light source that is pre-programmable and can be synchronized with a sensor device.